1. Field of the Invention
The present invention relates to a data transfer system in a device control apparatus for performing data transfer between a channel of a master device and a slave device such as a disk drive.
2. Description of the Related Art
In recent years, the difference in transfer capacities between channel interfaces between a CPU and a disk control apparatus and between the disk control apparatus and a disk device has been expanding. Therefore, it has been required to provide a data transfer system in which a synchronous transfer system for the same transfer rates and an asynchronous transfer system via a buffer for different transfer rates are properly combined.
In the conventional disk sub-system, there are various large and small combinations between channel transfer rates and disk transfer rates. In recent years, the difference of transfer capacities between channel interfaces and device interfaces has been expanding.
For instance, in an optical channel interface, transfer rate of 10 MB/s has been achieved. In contrast, the disk drives have transfer rates of 4.5 M B/s at the highest.
In order to accommodate such a difference in transfer capacities of such channels and disk drives to extract 100% of the transfer capacity of the channels, there has been proposed an asynchronous transfer system for performing a reading and writing operation through a data buffer in the disk control apparatus.
The asynchronous transfer system for transferring data through a buffer has a large overhead for each process. Namely, for each command of writing or reading, a preparation process and an ending process are necessary for writing and reading data to and from the buffer. The asynchronous transfer system, however, is advantageous when the transfer rates of the channel and the disk device are largely different, because the degradation of performance due to the overhead will not become significant by efficient use of the channel when it is free during the overhead process. Conversely, when the transfer rates of the channel and the disk device are almost the same, the asynchronous transfer system is not preferable because the overhead causes a delay, and the synchronous transfer system is preferable in this case.